It comes as North Korea test launched another missile over Japan on Friday following an earlier hydrogen bomb test which sparked a 6.3 magnitude earthquake.
Mr Mattis said he and his South Korean counterpart had discussed the idea of introducing nuclear weapons to the Korean peninsula but would disclose any further details about the military plans.
He added: “Yes there are. But I will not go into details.”
Mr Mattis also said diplomacy and sanctions were working and putting pressure on Pyongyang.
His comments follow a display of military might by the US over the Korean Peninsula as four F-35B stealth fighters and two B-1B bombers flew across the region as a warning to Kim Jong-un.
Dramatic photographs taken at the bombing range show the fighter jets zoom past overhead, before dropping their weapons into a mountain-side crater causing a huge fiery explosion.
The drills are the first flights since North Korea conducted its sixth and most powerful nuclear test on September 3 – along with staging another missile test over Japan, sending regional tensions soaring.
As part of today’s “routine” training exercise, US jets few alongside four South Korean F-15K jet fighters to drop MK-82, MK-84 and GBU-32 bombs on stimulated targets at the Pilseung Range in Gangwon-do.
After the drills, the F-35B jets and B-1B bombers returned to their bases in Japan and Guam respectively, sources said.
But the joint drills will be conducted “two to three times a month these days”, Defence Minister Song Young-moo told a parliamentary hearing on Monday.
The US will also sent an aircraft carrier strike group to join military exercises next month, according to South Korea’s defence ministry.
And South Korea and the US will also conduct a combined missile alert drill joined by Japan between late September and early October.
Earlier South Korea’s president Moon Jae-in vowed strong punishment on the North, adding the situation was the “most serious and urgent security issue in this region at the current moment”.
He added: “North Korea should realise that dialogue and cooperation, not nukes and missiles, are the only means to protect its security and guarantee a bright future.”
Moon had spoken Donald Trump earlier in a telephone call, where the pair agreed to exert stronger sanctions on the hermit kingdom following its nuclear and missile tests this week.
Park Soo-hyun, the spokesman for South Korea’s Blue House, said: “The two leaders agreed to strengthen cooperation, and exert stronger and practical sanctions on North Korea so that it realises provocative actions leads to further diplomatic isolation and economic pressure.”